According to a prior art image sensor chip, image signals from a plurality of photoelectric elements are serially output from an output pad without being amplified. A prior art image sensor comprising a plurality of such image sensor chips has an amplifying circuit formed on an amplifying circuit board separate from an image sensor chip board. The image signals from each of the image sensor chips are supplied to the amplifying circuit via a wiring pattern and a connector of the image sensor chip, and a wiring pattern of the amplifying circuit. After the signals are amplified by the amplifying circuit, the signals are serially output via the wiring pattern of the amplifying circuit, a connector and other components to an external circuit.
However, according to the prior art, the image signals from each of the image sensor chips are supplied to the amplifying circuit only after passing through the wiring pattern and the connector of the image sensor chip, and the wiring pattern of the amplifying circuit. Therefore, the signals are subject to noise, and it was difficult to obtain accurate image signals of an original image. Specifically, the image signals have a small voltage whereas the amplifying circuit has a large input impedance. As a result, noise can enter easily, and may become very large if there is a long path between the image sensor chip and the amplifying circuit. Especially, if there is such an apparatus as a laser printer located near by, the image signals can be badly affected by the noise from the laser printer.
This problem may be solved for example, by encasing the image sensor in a structure made of metal such as aluminum. However, this method of electromagnetically shielding the internal circuitry of the image sensor increases manufacturing cost of the image sensor.
Another option for solution may be providing each of the image sensor chips with a built-in amplifying circuit so that each image sensor chip can output amplified image signals.
With such an arrangement, however, the amplifying circuit built in each of the image sensors will have different offset from each other, making difficult to obtain accurate image signals of an original image. This problem is particularly serious in image sensing of a colored or gradated image because the difference in offsets can easily result in difference in the gradation or color.
This problem may be solved by adding a correction circuit for correcting the differences in the offsets. The correction circuit may be added to an image signal processing circuit which processes the image signals after the signals are output from the image sensor. However, this arrangement increases manufacturing cost of the apparatus which includes the image sensor.
Another option for solution may be providing one of the image sensor chips of each row with a built-in amplifying circuit so that image signals from all of the image sensor chips of that row can be amplified by this amplifying circuit before output to an external circuit.
This arrangement can solve the problem of the difference in offset within each row. In addition, there will be less problem of noise than in the case where the amplifying circuit is made separately on the amplifying circuit board. With this arrangement, however, two types of image sensors must be manufactured for mounting on an image sensor chip board. This decreases advantages of mass production in manufacture of the image sensor chips. Further, a more complex procedure must be used for mounting the image sensor chips on the image sensor chip board, increasing manufacturing cost of the image sensor.
As exemplified above, each of the three methods has advantages and disadvantages. Preferably therefore, any of the three methods should be selectively usable depending upon application of the image sensor. However, manufacturing different types of image sensors for each method will further decrease the advantages of mass production in the manufacture of image sensor chips, increasing further the manufacturing cost of the image sensor.